大大粒鑽石！

The ancient zircons crystals include even smaller diamonds

Tiny diamonds found in Australia suggest the early Earth was not a hellish world for as long as previously supposed, the journal Nature reports.

The miniature gems, from Jack Hills in the west of the country, are encased in zircon crystals that have been dated up to 4.25 billion years ago.

Scientists say their analysis of the diamonds suggests the planet had cooled sufficiently by then to form a crust.

This shell may even have been moving and exchanging material with the deep.

In other words, the diamonds could be the earliest evidence for plate tectonics, the theory used today to explain how the continents drift across the surface of the globe and rocks are recycled into the interior.

"We have compared these diamonds with known diamonds and so far it seems like the most similar diamonds are ultra-high pressure diamonds which form in a subduction process," explained Martina Menneken from Westfalische-Wilhelms University in Muenster, Germany.

"Today these form in plate tectonics surroundings. The implication is: did we have plate tectonics at this early stage of the Earth? I still think this is controversial; we cannot prove it and we need to do further research," she told the BBC's Science In Action Programme.

Rock survivors

Scientists describe the first phase in Earth history as the Hadean, when the planet would have begun its existence nearly 4.6 billion years ago as a sphere of molten rock circling the proto-Sun.

The traditional view was that it took several hundred million years for the Earth to cool enough for a crust to form and for oceans to condense out of a thick atmosphere.

That view has been challenged in the past 20 years with the discovery of zircons in Western Australia. These minute crystals - made of zirconium, silicon and oxygen, among other elements - are the tough remnants of ancient rocks that have long since disappeared. Today, these crystals are incorporated into more recent rocks, such as those at Jack Hills.

A seething, molten sphere: How quickly did things cool down?

The new analysis by Menneken and colleagues looks at the diamond inclusions in the zircons.

It is not clear how the diamonds got into the zircons. Normally, diamonds will form under intense pressure, from deep burial or perhaps from meteoroid impacts.

The team says that from its examination, deep burial looks the more obvious contender - and that supports the notion of a crust that was re-cycled deep within the Earth.

Geoscience 'presents'

This suggestion is problematic, however, for some commentators, who point out that the zircon crystals themselves show no signs of being exposed to very high pressures.

Dr Ian Williams, of the Australian National University, says this conundrum could in fact lead some to take diamond analysis as support for a hotter early Earth.

This would undermine other inferences about the zircons, he said, in particular that they formed at lower temperatures and at shallower depths within the Earth.

What is certain, say researchers, is that the tiny zircons - they are about 0.3mm across - continue to give fascinating insights into the Earth's beginnings.

"Any information about the very early Earth is fantastic; it's like a Christmas present for geoscientists," said Dr Martin Van Kranendonk, a senior geologist with the Geological Survey of Western Australia.

"[The Nature study] provides a new constraint for geoscientists to consider how the Earth formed into the planet it is today. It's another piece in the early Earth puzzle."

附送一篇山林大火　＝．＝　　　http://news.bbc.co.uk/2/hi/science/nature/6966822.stm

ANALYSIS By Richard Black Environment correspondent, BBC News website

Fires are being fought in Greece and the Balkans

And what about all that carbon dioxide? A significant impact on climate change?

Despite the long history of burning bushes around the Med, there is much for scientists still to discover about the ecological effects of fire.

"In terms of processes like degradation in these burnt areas, we don't have a clear idea," says Paulo Barbosa from the European Commission's Institute for Environment and Sustainability.

"We know that when you have very large fires and have heavy rains afterwards, that can induce erosion and soil loss.

"But... if we talk about shrubland vegetation, it should take about three to four years to return to conditions before the fire. With forests, we can talk about 10, 20, 30, even 40 years to return to their original conditions."

Uniform weakness

Forests in many regions of the world including the Mediterranean have evolved to withstand fire, even to flourish in it.

It's not just about putting trees back for the sake of the trees. Tamas Marghescu, IUCN

But the ecological impact depends on the heat of the fire. A light blaze may leave root systems unscathed, whereas a major conflagration can kill them off.

Recent changes, Dr Kirby suggests, might make Mediterranean forests a little more vulnerable than they were.

"In other Mediterranean countries, impacts have been exacerbated through large plantations of uniform, flammable species such as conifers and eucalypts. You'll have the risk that species will be isolated in unburned patches and not be able to recolonise."

Thinking ahead

The World Conservation ｕｎｉｏｎ (IUCN) is assembling a task force to research and restore the newly burned regions of Greece, and other affected countries such as Cyprus and Serbia, when the flames have died.

"We're intending to focus on protected areas and national parks, which is something we are specialised in and where we could give some assistance to affected states," says Tamas Marghescu, IUCN's regional director for Europe.

"What's very important is that it's not just restoration of the ecosystem, but of ecosystem services important for human beings.

Dry, hot and windy conditions contribute to fires spreading

Which trees to put back is a big question. Climate change promises to make much of the Mediterranean region warmer and drier, to reduce the availability of water which is already scarce in some areas, notably parts of the Iberian peninsula.

Does it make sense, then, to replace burnt trees like for like?

Mr Marghescu thinks not. He believes the fires represent an opportunity as well as a threat - an opportunity to bring the demands of the forest more into line with what the future climate will provide.

Trees vulnerable to fire and drought could be replaced by other species more resistant to flames and more thrifty with water.

To those who might protest at a deliberate changing of the ecosystem, he replies: "An ecosystem is not something rigid; it is always moving in some direction, and there is always a change of species happening.

"We just intending to assist that adaptation, so we do not have a collapse of the ecosystem through climate change."

Even picture

The Greek fires of 2007 were ｃ ｒｅａｔｅd by a combination of people deciding to light them, and meteorological conditions that made spontaneous ignition and spread more likely.

As in Spain and Portugal in 2003 and many other summers, high temperatures, low humidity and strong winds created and fanned innumerable flames.

Greece battles against devastating forest fires In pictures

The average lost around the Med each year, at 450,000 hectares, is tiny against this backｄ ｒｏｐ.

But with European temperatures apparently increasing decade by decade, are fires on the increase?

Apparently not, says Paulo Barbosa, who runs the European Forest Fire Information System (Effis).

"There is great variability," he says. "Last year only 380,000 hectares burned, while in 2003, almost a million burned.

"So you see extreme events, but it's hard to say there's a trend."

So the disappearance of European forests is unlikely to be a major contributor of greenhouse gases in the future.

In fact, European forest cover is increasing, by about half a percent each year. Spain alone is adding nearly 300,000 hectares to the mix annually.

Globally, forest loss and land use change is a major driver of increased greenhouse gas concentrations, though the Intergovernmental Panel on Climate Change (IPCC) ranks its importance second behind fossil fuel use.

If previous years are a guide, this summer's fires in Greece, Cyprus and the Balkans will fade with the autumn rains - unlike parts of Indonesia, for example, where some forests contain peat beds which have smouldered for years.

The rains should put a dampener on the idea that the flames, despite their impact on the lives of those directly affected, herald long-lasting environmental change.